They were originally published between 1912 and 1965 and reveal much of the scientific thinking of the time period, a time when evolutionary theory was budding as the enlightened path. (Charles Darwin lived from 1809 to 1882.) Originally, the ideas of evolution began as a theory that the races evolved differently; white man descended from the line of the more intelligent monkeys, while black man was from the line of the great apes, who were less intelligent, and the oriental man was from the orangutan line.
These racist ideas abound in the Tarzan series, where Tarzan is shown to be supreme over the black men of the jungle because their brains aren't as well developed and he has a greater imagination. The black man's brain is seen as superstitious and less developed, as clearly stated evolutionary teaching. The whole idea of Tarzan was that white man is superior because God made him superior. The rich man is rich because he is superior to the poor. The position was taken in Tarzan that if you took the baby of an English Lord, abandon him as an orphan in the jungle, then he would rise to be lord of the jungle, simply because the aristocracy was in his blood. He was destined for higher thinking and better equipped to dominate. And, indeed it was better for all concerned if he dominates them. They will benefit from his lordship.
Of course, we know this is hogwash now. All of it. So, why are we still teaching evolution/ science fiction in schools?